1. Size of eggs.
a. not correlated that large female parasitoids deposit large eggs.
b. egg size is related to the number of ovarioles and to the egg deposition rate.
(1). proovigenic females with large numbers of ovarioles and a high deposition rate tend to
produce small eggs.
(2). e.g., females of all known species of Trigonalidae (Hymenoptera) lay up to 10,000 eggs at the
rate of 5,000 per day!
a. the majority of endoparasitoid Hymenoptera have semi-transparent and unsculptured chorions.
b. among ectoparasitoid Hymenoptera, chorions may be adorned with tubercles, spines or ridges.
3. Egg Types
a. hymenopteriform egg
(1). ovoid to spindle-shaped in outline and are smoothly rounded at both ends.
(2). chorion is either smooth or variously sculptured.
(3). deposited internally, externally or apart from the host.
(4). of general occurrence in parasitoid Hymenoptera, but also found in some families of parasitoid
b. acuminate egg
(1). elongate, tapering to a sharp point at one or both ends.
(2). chorion is smooth.
(3). found largely among Ichneumonidae and Braconidae, parasitoids possessing long ovipositors
for reaching hidden hosts in galls, galleries, wood tunnels, etc.
c. stalked egg
(1). tube-like extensions at one end.
(2). generally found in parasitoid Hymenoptera and a few Diptera.
(3). may be deposited within, upon or apart from the host.
d. encyrtiform egg
(2). deposited internally.
(3). one collapsed "bell" and a portion of the stalk that connects the two, remain protruding from
the ovipositional puncture.
(4). the projecting structures bear a longitudinal rib along one side called the aeroscopic plate that
functions in larval respiration.
(5). found in many genera of Encyrtidae.
e. pediculate egg
(1). one end penetrates the host integument and is variously twisted, expanded or knotted to serve
as an anchor for the externally projecting egg body.
(2). found in Agriotypidae, Ichneumonidae and Eulophidae.
f. macrotype egg
(1). large, oblong and ventrally flattened.
(2). deposited externally.
(3). found only in Tachinidae.
g. microtype egg
(1). minute, oval, ventrally flattened.
(2). deposited on foliage apart from hosts and hatch only upon being eaten by the host.
(3). common in Tachinidae and Trigonalidae.
h. membranous egg
(1). chorion is extremely delicate.
(2). deposited either internally or externally.
(3). found in Tachinidae and Sarcophagidae.
i. acroceriform egg
(1). pear-shaped and darkly pigmented.
(2). the smaller end bears a well-defined circular cap which is forced off at eclosion.
(3). found in Cyrtidae (Diptera).
Usually only a single parasitoid is produced per egg in monoembryony. Sometimes the egg develops
a. has developed independently in four hymenopterous families: Braconidae, Encyrtidae,
Platygasteridae and Dryinidae.
b. also present in a few species of Strepsiptera.
c. the number of individuals arising from each egg is extremely variable, ranging from two to 2,000
as in the genus Litomastix (Platygasteridae). The number is apparently directly proportional
to the size of the mature host larva.
d. host preference is shown, as, e.g., the polyembryonic Braconidae and Encyrtidae only parasitize
Lepidoptera; whereas polyembryonic Platygasteridae parasitize hosts in the dipterous family
e. restricted parasitoid genera: only in the Encyrtidae is more than one genus in a family known to
f. host stage attacked: all polyembryonic Encyrtidae and Platygasteridae oviposit in the egg of their
hosts and complete their development in the mature host larva or pupa. Thus, they are all
either egg-larval or egg-pupal parasitoids.
g. Sex: the parasitoid brood from a single host may be all of one sex or mixed.
h. Distinction from Gregariousness
(1). polyembryonic species oviposit in the egg or very young host larva, with parasitoid maturity
occurring in the mature host larva or pupa.
(2). exceptionally large numbers of progeny usually develop in a single host.
(3). simultaneous development and emergence of the brood.
(4). a portion of the broods consist of one sex only, and the mixed broods show widely varying
i. Polyembryonic development results in an increased reproductive capacity, but does not
necessarily confer a corresponding increased efficiency as a natural enemy. Polyembryony
may, instead, be viewed as an effort on the part of the parasitoid to overcome certain
unfavorable factors in its environment.
j. Relatively few polyembryonic species have been known to function effectively as biological
control agents. However, as in the case of the navel orangeworm, Amyleois transitella, they
may work in concert with other parasitoids to produce effective biological control.
B. First-Instar Larvae
The most distinctive and variable stage in the life cycle of many entomophagous parasitoids and
a. Planidium-type larva
(1). Greek word meaning "diminutive wanderer."
(2). all Eucharitidae and Perilampidae and males of Aphelinidae; also dipterous Cyrtidae and many
(3). spindle-shaped, heavy sclerotized, possess sensory organs and equipped for locomotion by
means of thoracic or caudal ambulatory setae; or by vigorous twisting, jumping or looping
(4). can survive weeks or more without feeding.
(5). they arise from eggs that are deposited apart from their hosts.
(6). upon hatching, they search for or otherwise contact their hosts. They are strongly attracted
to any moving object and attach themselves to passing hosts or to nonhost carriers which
then carry them to their hosts.
b. Triungulinid-type larva
(1). the counterpart of the planidium larva but found in Strepsiptera, and coleopterous Meloidae
(2). similar in all respects to planidium larvae, with the exception that they possess segmented legs
c. Sacciform-type larva
(1). body in bag-like form, lacking apparent segmentation and lacking a tracheal system.
(2). develop only internally.
(3). found in certain Dryinidae, Trichogrammatidae and Mymaridae.
d. Teleaform larva
(1). body segmentation also not apparent.
(2). cephalothorax and abdomen separated by a deep constriction.
(3). mandibles are very large.
(4). abdomen sub-spherical and bears a long, blade-like process posterio-ventrally.
(5). internal larval forms found in Scelionidae.
e. Mymariform larva
(1). spindle-shaped and indistinctly segmented.
(2). head conical.
(3). body segments ringed with long spines.
(4). last abdominal segment greatly elongated and tail-like.
(5). internal larval forms found in certain Mymaridae and Trichogrammatidae.
f. Cyclopiform larva
(1). cephalothorax larger than abdomen.
(2). mandibles very large.
(3). abdomen tapers posteriorally and its last apparent segment is usually forked.
(4). the majority of Platygasteridae; all internal.
g. Eucoiliform larva
(1). distinguished by the paired, fleshy ventral processes on each thoracic segment. Also, sharply
tapered, often tail-like abdomen.
(2). internal; found in certain Cynipidae.
h. Mandibulate larva
(1). distinct segmentation and large, broad, somewhat flattened, heavily sclerotized heads that are
armed with large sickle-shaped mandibles.
(2). internal forms; found in many Ichneumonidae, Braconidae, Serphidae and Diapriidae.
i. Microtype larva
(1). minute in size.
(2). integument delicate.
(3). each thoracic segment bears a series of heavy spines or hooks.
(5). hatch from microtype eggs of the Trigonalidae and many species of Tachinidae.
j. Muscoidiform larva
(1). commonly called "maggots."
(2). found in the suborder Cuyclorrhapha of the Diptera.
k. Hymenopteriform larva
(1). larvae spindle-shaped to spherical in outline.
(2). usually 12-13 body segments distinguishable.
(3). integument bare or studded with sensory setae and cuticular spines.
(4). includes both internal and external forms and is of general occurrence in the Hymenoptera.
l. Agriotypiform larva
(1). bodies of these larvae bear a transverse row of long, heavy spines dorsally on each segment.
(2). last abdominal segment bears two, long and slender, sharply-pointed and heavy sclerotized
(3). external forms found only in Agriotypidae.
m. Vesiculate larva
(1). similar to hymenopteriform type, except that the hindgut protrudes posteriorly as an enlarged,
(2). internal only.
(3). many Braconidae.
n. Caudate larva
(1). distinctly segmented, usually somewhat elongate.
(2). last abdominal segment is modified into a fleshy, tail-like organ.
(3). internal only.
(4). found only in many Ichneumonidae and Chalcidoidea.
C. The greatest variation in larval form occurs in the first instar. Development thereafter tends to
converge towards the hymenopteriform larva in parasitoid Hymenoptera and towards the muscoidiform larva in the
The intermediate and last-instar larvae of ectoparasitoid Hymenoptera generally do not undergo great
changes in form as they progress towards maturity.
However, endoparasitoids, and those species in which the eggs or larvae are deposited apart from their
hosts, usually undergo conspicuous modifications during their larval development. These changes in
larval form may be completed by the second instar or the transition may be more subtle, with progressively
more simplified larval forms interposed between the first and last instar.
The intermediate stages of both dipterous and hymenopterous parasitoids usually resemble the last instar
in form. The greatest change usually takes place at the first molt among parasitoid species that possess
the most highly specialized primary larvae, namely the planidium, cyclopiform, teleaform, agriotypiform
and mandibulate types. Here, by the second instar the larvae are either hymenopteriform or are very close
to the same.
D. Special Larval Types
1. In certain Cynipoidea having eucoiliform primary larvae, the 2nd instar is called polypodieform. This
unique intermediate stage larva has a distinctly segmented body, several anterior abdominal segments
of which each bears a pair of ventrally-directed, fleshy lobes.
2. Another distinctive 2nd instar larva is the histriobdellid type found among Mymaridae egg parasitoids
that have sacciform primary larvae. This intermediate type has a cylindrical body that is interrupted
by 6 annular constrictions. The head bears a pair of large, slender curved mandibles; and both the head and
the last apparent body segment each bear a pair of fleshy lobes.
D. An interesting phenomenon associated with the larvae of parasitoid Hymenoptera is the fact that the
hindgut is not excretory in function until the prepupal molt is about to occur. Until this time the hindgut
ends blindly and may occupy much of the body cavity of the larva, serving as both an organ of digestion
and storage. At the time of the prepupal molt, all fecal material accumulated and stored in the hindgut
during larval feeding is released at one time, forming what is called the meconium.
II. SEX DETERMINATION AND PARTHENOGENESIS
A. In Hymenoptera, sex determination follows what is called Dzierzon's Law. Dzierzon was a
Silesian priest who lived around 1845.
1. males are derived from haploid, unfertilized eggs; females from diploid, fertilized eggs.
2. diploidy is brought about in either of two ways:
a. as a modification of meiosis in the ovary.
b. by fertilization of the haploid egg at the moment of oviposition, which changes the sex of the egg from
male to female.
B. Genetics of Sex Determination
a. originally thought to be like Drosophila (e.g., males = X; females = XX)
b. Petrunkewitsch (1901) believed that gonads were diploid even though the male body was
c. Castle (1903) considered differential egg maturation.
d. Nachtsheim (1913) proposed differential egg maturation directed by the presence or absence of
a sperm nucleus.
e. P. W. Whiting (1933) developed an early theory of multiple alleles.
f. P. W. Whiting (1943) perfected the multiple allele theory
xa, xb, ..., xi -- any heterozygote (diploid), xa/xb, xc/xd, etc. is female.
xa, xc, etc. -- any azygote (haploid) or homozygote, xa/xa, xb/xb, etc. is male.
g. Cunha and Kerr (1957) developed the theory of a series of male-determining genes in balance with a series
of female-determining genes. The female-determining (FD) genes would be additive in their effect, whereas
the male-determining (MD) would not.
C. In most Hymenoptera, the spermatheca functions as a sex-changing organ. There are two principal ways
in which this sex-changing process operates.
1. In Braconidae, Ichneumonidae and aculeate Hymenoptera (bees and wasps), the process begins when
stimuli from the oviposition site activate the sperm stored in the spermatheca. Prior to this necessary stimulation
by host contact, the stored sperm are inactive (incapable of locomotion). Once the sperm are activated, each
time an egg passes down the oviduct, it stimulates several sperm to be emitted, which enter the egg through
the micropyle and fertilization results.
2. In Chalcidoidea, a secondary sex changing mechanism is present following sperm activation. This is the
control of sperm emission from the sperm duct of the spermatheca. The passage of the egg down the oviduct
usually stimulates the emission of but a single sperm. However, another stimulation from the oviposition
site may secondarily stimulate a muscular contraction that closes the aperture of the sperm duct, so the egg remains unfertilized and male at deposition.
D. Three types of parthenogenetic reproduction
a. obligatorily parthenogenetic.
b. each generation consists almost entirely of females; males are rare.
c. the progeny of the virgin female are necessarily uniparental.
a. both males and females are produced parthenogenetically.
b. both males and females are uniparental.
c. the same as thelytoky except that there are more males present in the population.
a. the majority of parasitic Hymenoptera are arrhenotokous.
b. females are derived from fertilized eggs as a result of the spermatheca operating as a sex-
c. in species exhibiting arrhenotoky, the females, therefore, are usually biparental and the males
III. HOST SELECTION
A. Analyses of the manner in which entomophagous insects find their hosts and the bases for their host
preferences, as with phytophagous insects, currently are subjects of active entomological inquiry.
B. Host parasitoids in nature attack several host species, although a few monophagous species are known.
C. No parasitoid appears to be completely indiscriminate, however, in its choice of hosts. In nature only a
fraction of the species on which development is actually possible are attacked by any one species.
D. Definite host preferences are expressed by various groups of parasitoids.
1. most parasitoids of Scarabaeidae larvae are in the hymenopteran families Scoliidae and Tiphiidae.
2. egg parasitoids are Trichogrammatidae, Mymaridae and Scelionidae.
3. parasitoids of gall midges, Cecidomyiidae, are Platygasteridae.
4. in the laboratory, however, spatial and temporal barriers which separate parasitoids from their potential
hosts in nature can be removed. Parasitoids can be bred in numbers on unnatural or factitious hosts. This
is actually practiced in the mass-rearing of beneficial parasitoids for biological control.
Example: the oriental fruit moth parasitoid, Macrocentrus ancylivorous, can be mass-reared on potato
tuberworm larvae, although this host/parasitoid relationship never occurs in nature. Similarly, synanthropic
fly parasitoids in the genus Muscidifurax can be reared on Drosophila in the laboratory, which greatly
stunts the adults which emerge. In nature Drosophila have never been found parasitized by this genus.
5. In the mid-1930's, the steps involved in host selection were discovered by Laing, Salt and Flanders.
a. Salt: Step I = ecological selection, where the parasitoid is brought into contact with its host; Step II =
psychological selection, where the host is accepted once contact is made; Step III = physiological selection,
where the suitability of the host as a food source is determined.
b. Laing: parasitoids find the environment of the host first, then the host itself.
c. Flanders: divided Salt's ecological selection into host-habitat finding and host- finding. The third and
fourth steps are host acceptance (equivalent to Salt's psychological selection) and host suitability (equivalent
to Salt's physiological selection).
(1). host-habitat finding = used to describe the process by which entomophagous insects orient to various
environmental stimuli characteristic of the habitats frequented by their prey.
(2). host-finding = describes the either random or nonrandom encounter of the prey individuals by the
parasitoid within its host's habitat.
6. Summary of Procedures in Parasitization
a. Host habitat finding represents the initial step in the chain-like series of events by which any host/parasitoid
relationship is maintained. A parasitoid initially detects certain habitats as those more likely to be frequented
by its host, even though those habitats at that particular time may not contain the host.
Example 1: an ichneumonid parasitoid Idechthis canescens is attracted by the odor of oatmeal, even though
its host, the larva of the Mediterranean flour moth, is not present.
Example 2: a chalcidid parasitoid of ant lion larvae, Stomatocerus rubrum, is attracted to sand and actively
explores any small depressions on the sand surface.
Example 3: Nasonia vitripennis is attracted to carrion that contains blowfly larvae. Either carrion or blowflies
alone are not attractive.
Example 4: Spalangia and Muscidifurax species are attracted to accumulated garbage or animal wastes in
which they find muscoid puparia as hosts.
Example 5: plant species may also prove strongly attractive to a species of parasitoid even though suitable
phytophagous hosts may not be present. On the other hand, parasitoids may ignore suitable hosts feeding
on plants which hold no attraction for the parasitoid. One notable example is exhibited by Pimpla ruficollis,
an ichneumonid parasitoid of the European pine shoot moth. Here sexually immature females are unresponsive
to the odor of pines, but sexually mature females are strongly attracted by pine odor.
Once a parasitoid has reached its host's habitat, it attempts to locate a host individual. Considerable research
shows that various combinations of random and directed movements (taxes) are involved. Chemotactic,
phototactic, hydrotactic and geotactic responses, among others, all seem to play a part in the host-finding
process. These responses are variously modified by olfactory, visual and other physical stimuli that
characterize a parasitoid's prey.
Once physical contact has been made, only after receiving a proper combination of stimuli will further
behavioral responses be triggered, resulting in acceptance of the prey; i.e., host-feeding and/or oviposition.
The stimuli for attack are known to involve, among other factors, host odor, host size, host location, host
shape and even host motion.
It is well known that many parasitoids have the ability to discriminate between parasitized and healthy hosts
and thus avoid superparasitism. This differentiation may result from an odor left on the host by the parasitoid
that first contacted, the so-called spoor effect. The parasitic Hymenoptera, as a rule, are more discriminatory
than the parasitic Diptera in their selection of hosts. Among predacious species, host specificities range
from those which are nearly monophagous (e.g., Rodolia) to those which are highly polyphagous
(Geocoris spp.). Relatively speaking, a greater proportion of parasitoids than predators exhibit monophagy.
The fact that a parasitoid has found a potential host within its respective habitat and has oviposited in or
upon the same, is no assurance that all criteria for maintaining a host-parasitoid relationship have been met.
The host individual selected may prove unsuitable for parasitoid development. In other words, oviposition
is no assurance of host suitability if the host individual proves to be resistant or otherwise unsuitable for
A host may be unsuitable as follows:
(1). for physical reasons (too small, too thick).
(2). for nutritional reasons.
(3). biological reasons: the host may be killed by the ovipositing female following host-feeding or mutilation
. The host may move and dislodge externally-attached parasitoid eggs or larvae. The host may molt and thus
shed parasitoid eggs attached externally to the cast exuvium. Also, internally laid eggs and endoparasitoid
larvae may be encapsulated by phagocytes. Phagocytes are blood cells that gravitate to and either ingest
or surround foreign bodies that are introduced into the haemocoel of a host insect. The process is called
Evidence exists that formerly susceptible host populations may become resistant to parasitoid attack. Cases
are also known where otherwise normal hosts are rendered unsuitable by the host plants on which the host develops.
This fifth category in the host selection process was proposed by Dr. Bradleigh Vinson of Texas A. & M.
University to account for cases in which parasitism changes the host physiologically, causing it to behave
in a different manner. It does not have anything to do with regulation of host numbers.
7. Manner and Place of Oviposition
a. Obviously, those species that oviposit merely in the vicinity of hosts or randomly within their host's
general habitat, are not exercising as much discrimination as those parasitoids in which host-selection behavior
is developed to the degree where a specific host organ or location on a host serves as the oviposition site.
b. Many species of Diptera and a few parasitic Hymenoptera, oviposit in habitats frequented by their hosts,
but apart from any host individuals that may be present. These parasitoids may lay their eggs more or less
at random upon plant foliage or other plant parts, and host contact is made when those eggs are subsequently
ingested by their plant-feeding hosts. The eggs of some Hymenoptera hatch into small, motile larvae which
usually can live without food for long periods of time and which attach themselves to passing host individuals.
Some dipterous parasitoids are viviparous with the eggs hatching within the parasitoid female the subsequently
larviposit within the vicinity of, but apart from, their hosts.
c. The eggs of many species of dipterous and hymenopterous parasitoids are deposited on the host. The
larvae, after hatching, variously feed either externally as ectoparasitoids or enter the host and develop as
endoparasitoids. The eggs of such parasitoids may either be glued to the host integument or anchored in
place by peg-like extensions of the chorion which penetrate the host's integument.
d. It can generally be said that hosts living in exposed situations, such as leaf-skeletonizing larvae, tend to
be attacked by endoparasitoids; whereas, hosts living in protected situations, such as galls, tunnels, galleries,
mines, or in puparia or cocoons, tend to be attacked by ectoparasitoids. It follows that parasitoids of exposed
hosts generally oviposit within their hosts. These eggs may simply be thrust into the host's haemocoel and
left to float free in the blood, or the eggs may be inserted into specific host organs.
IV. POLYGENES AND EXTRANUCLEAR INHERITANCE
It is generally agreed that most, if not all, behavior in animals is governed by polygenic loci. Yet, due to
inherent difficulties with studying polygenic inheritance, data has been difficult to obtain. Considerable
progress has been made with a parasitic hymenopteran genus Muscidifurax in the late 1980's. It was learned
that quantitative behavior associated with gregarious oviposition (>one individual developed per host)
and fecundity in the South American parasitoid Muscidifurax raptorellus Kogan & Legner was controled
by such polygenes (Legner 1987a, 1991). This system provides insights into the true nature of polygenic
loci. Simply the data derived suggest that even animals which do not show the particular trait (e.g., high
fecundity, gregarious development, aggressiveness, tallness, shortness, integument color, etc.) may have
all loci present for the maximum expression of such traits, but in some cases not all loci are turned on or
activated. In hybrid cases only a certain number are turned on.
In the M. raptorellus system, inheritance of polygenic traits is accompanied by some unique extranuclear
influences which cause changes in the oviposition phenotype of females (Legner 1987a,b; 1988a). Males
are able to change a female's oviposition phenotype upon mating, by transferring an unknown substance
(Legner 1987a, 1988a,b). Females with a solitary genotype produce larvae with gregarious development
after mating with males possessing the gregarious genotype, and females with the gregarious genotype
produce larvae with reduced gregarious behavior after mating with males of the solitary genotype. The
intensity of this response is different depending on the respective genetic composition of the mating pair
(Legner 1989a). Thus, the genes involved, by regulating phenotypic changes in the mated female, cause
partial expression of the traits they govern shortly after insemination, and before being inherited by
resulting progeny (Legner 1987a, 1988a, 1989a).
Maternal inheritance of extranuclear substances as discussed by Legner (1987a) and Corbet (1985)
seemed a possibility for the passage of traits to offspring. However, observations of linear additivity
of the traits and variance changes in hybrid versus parental generations and relatively constant daily
expressions of behavior in F1 and backcrossed populations, point to chromosomal inheritance (Legner
1987, 1988a, 1989a,c). Chromosomal inheritance of gregarious behavior was substantiated further by
the formation of recombinant males, thereby enabling estimations of the number of active genetic loci
governing gregarious development (Legner 1991a,b).
The inheritance scheme in Muscidifurax is fundamentally important to an increased understanding of the
genetic of Hymenoptera. Therefore, the kind of genes and their mode of inheritance deserve distinction.
Genes of this sort that are able to cause partial phenotypic changes in mated females before being inherited
by their progeny have been termed wary genes because they, or their precursors, are tested in the
environment in an attenuated manner before being inherited by the offspring (chromosomal inheritance).
Whether such genes possess chemical precursors capable of changing the female's phenotype, or are
inherited extranuclearly after mating is unknown.
The behavioral change after mating is permanent, and there is no switchback in behavior following a
second mating with the opposite parental male. This suggests that a relatively stable molecule like DNA
may be present and becomes permanently active after a first mating. Speculations also have considered
microorganisms, male accessory gland fluids, and behavior-modifying chemicals, such as prostaglandins,
responsible for the behavioral changes after mating. Nevertheless, signals are sent to a female from the
male within hours of mating, probably via the sperm or seminal fluid. These signals express the code of
the genes themselves. The genes present in the male are then inherited by the progeny in a typical
polygenic inheritance. Because inheritance of such genes seems to occur in a stepwise manner, the entire
process might be termed accretive inheritance (Legner 1989a).
In the process of hybridization, wary genes may serve to quicken the pace of evolution by allowing natural
selection for nonlethal undesirable and desirable characteristics to begin to act in the parental generation.
Wary genes detrimental to the hybrid population might thus be more prone to elimination and beneficial ones
may be expressed in the mother before the appearance of her active progeny. If wary genes occur more
generally in Hymenoptera, their presence might account partially for the rapid evolution thought to occur i
n certain groups of Hymenoptera (Hartl 1972, Gordh 1975, 1979), and possibly the quick adaptation and spread
of Africanized honey bees in South America as discussed by Taylor (1985) and Legner (1989d) [Please
see Expanded Research on this subject].
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Flanders, S. E. 1937. Habitat selection by Trichogramma. Ann. Ent. Soc. Amer. 30: 208-10.
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Legner, E. F. 1987b. Further insights into extranuclear influences on behavior elicited by males in the genus Muscidifurax [Hymenoptera: Pteromalidae]. Proc. Calif. Mosq. & Vect. Contr. Assoc. 55: 127-30.
Legner, E. F. 1988a. Muscidifurax raptorellus [Hymenoptera: Pteromalidae] females exhibit post mating oviposition behavior typical of the male genome. Ann. Ent. Soc. Amer. 81: 524-27.
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